In conventional foil-blowing apparatus, an extruder extrudes a continuous stream or strand of hot (molten) thermoplastic synthetic resin through an annular nozzle so that the tubular flow which emerges can be then expanded by blowing it, e.g. with air, to reduce the wall thickness and form a foil. The foil tube which is thereby produced, stabilizes upon cooling and can be collapsed and coiled, if desired.
The principles of such foil blowing of tubes and the general principles of control of the foil blowing process can be deduced from my commonly owned U.S. Pat. No. 4,189,288.
It is known that temperature deviations at various regions around the periphery of the mass flow of the synthetic resin material emerging from the annular nozzle can be compensated by subdividing the annular nozzle into a plurality of sectors which can be independently controllable as to temperature, these sectors being heated, cooled or selectively subjected to either a temperature raising or a temperature lowering operation.
In general this control is effected by measuring the wall thickness of the blown tube at different regions around its periphery and comparing these measurements with set point values, thereby generating error or control signals which can be used to regulate the temperature of the so called tempering sector.
The foil blowing head can rotate, can angularly oscillate or can be stationary.
The tempering sectors usually all have the same circumferential or arc length. They can be provided with heating and/or cooling devices (German patent document - Open Application - DE-OS No. 21 40 194). In addition, the blowing head can be provided with an autogeneous heat exchanger (German patent document - Open Application DE-OS No. 32 11 833), which can comprise an annular gap surrounding the blowing head axis and extending substantially parallel to the mass flow of the thermoplastic synthetic resin, the gap being partly filled with a heat exchange medium which is vaporizable or condensable at the operating temperature and is hermetically sealed.
Naturally the process with which the various regions of the mass flow can be controlled will increase with the number of sectors into which the annular nozzle is divided. Customarily four to eight sectors are provided.
In German patent document - Open Application DE-OS No. 30 02 293, there is described a control process in which a state parameter of the mass flow is measured, namely, the thickness of the foil tube, utilizing a thickness measuring device which is located downstream of the usual calibrating device and thus comparatively far from the annular nozzle and even from the expansion region of the apparatus. Such a thickness detector is likewise provided in the aforementioned U.S. patent.
As a consequence of the distance between the expansion region and the thickness measuring device or between the annular nozzle and the latter device, there is a significant response time between the initiation of a control operation and its measurable effect.
Furthermore, fluctuations between thickness maxima and thickness minima can be considerable in such control systems and thus around the circumference of the resulting foil the thickness tolerance is considerable. There is, furthermore, little capacity with the earlier systems to so coordinate the control of the individual sectors as to minimize the differences between thicknesses resulting at the various regions.
In the measurement techniques which have been used, the approach has generally been to utilize iterative or incremental elimination of a deviation from a set point value which, because of the response delay and the nature of such iterative or incremental approach to a set point value has made it difficult to obtain products with low thickness tolerances.
In fact, when rotary annular nozzles or angularly oscillating nozzles were employed, even the measurement of the thickness proved to be difficult and this was also the case because of interference with the thickness measurement by the means for the flattening of the foil tube.